Choline hydroxide, choline base and in short also “cbase”, are terms which are used in this document interchangeably. Choline hydroxide or choline base is also known as 2-hydroxyl ethyl trimethyl ammonium hydroxide or under IUPAC nomenclature rules 2-hydroxy-N,N,N-trimethylethanaminium. The substance is a strong yet organic base, which is an important element for its selection into many end-use applications. Choline hydroxide has applications in the production of other choline salts, for example, by neutralization with an appropriate acid and in applications where a strong base containing very low levels of inorganic ions is needed or where only very low levels can be tolerated. Choline hydroxide is important in a range of applications, such as in the manufacturing of electronics.
Choline hydroxide may be manufactured in a variety of different ways. For example, choline hydroxide may be produced from choline halides (e.g. choline chloride), choline hydroxide may be formed by treating choline sulphate with Ba(OH)2, or choline hydroxide may be produced from a direct reaction.
Choline hydroxide may also be produced by the direct reaction of trimethylamine (TMA), water, and ethylene oxide (EO). U.S. Pat. No. 2,774,759 discloses in example 2 the reaction of 236 parts of a 25% aqueous TMA solution with 40 parts of EO. The mixture is stirred until the reaction is substantially complete, while the reaction temperature is kept below about 30° C. Unreacted TMA is removed under vacuum at about 45-55° C., according to U.S. Pat. No. 2,774,759 leaving cbase in a 40-45% aqueous solution. Assuming ideal 100% selectivity in the reaction and in the TMA removal, one may calculate a product containing 40.65% wt cbase in water. The applicants repeated this experiment, found that the reaction is very slow under these conditions and that it was difficult to remove the excess TMA from the reaction product. The applicants obtained a cbase solution containing 38.5% wt choline hydroxide and 2.2% wt higher-ethoxylated by-products.
This direct method has the advantage of being much more atom efficient as compared to other methods, such as those involving a choline halide starting material. However, the direct reaction of EO and TMA in the absence of a strong acid (e.g. HX) also has some disadvantages.
The ethoxylation of TMA is typically performed in batch mode. Typical is the use of so-called loop reactors, a.k.a. pump-around reactors, whereby the reaction mixture is continuously pumped around from the reactor vessel over a heat exchanger, in order to effectively remove the reaction heat and to keep the reaction temperature low. In order to avoid excessive EO partial pressures, the EO is typically added gradually as the reaction proceeds. In order to drive the consumption of TMA towards completion, an overall molar excess of EO is typically supplied. Any excess of EO, however, will be consumed in O-ethoxylation and form the less desired O-ethoxylates as by-products. When the reaction is approaching completion, it may take up to 10 moles of EO in order to convert 1 further mole of TMA.
A first problem of this reaction path is mainly due to the nature of the trimethylamine (TMA) reactant. Firstly, it is fairly volatile, having an atmospheric boiling point of about 3° C. TMA has a strong and unpleasant fishy odour, and its smell threshold in air is as low as 2 parts per billion (ppb, 10−9). Vapour by-product streams containing TMA must therefore be incinerated before release, and this should be done at high temperatures to avoid formation of nitrosamines. This poses particular problems when the process involves vacuum conditions. These properties of TMA further impose that the choline product should end up being substantially free of unreacted TMA reactant. Removal of any remaining TMA from the reaction product by stripping with inert gas is unpractical because of its low atmospheric boiling point, meaning it is very difficult to condense from a mixture with an inert gas.
Another problem with the nature of TMA is that it has a limited solubility in water. Excessive presence of TMA will lead to the formation of a separate liquid phase, and not lead to a higher presence of the TMA reactant in the water phase where the reaction occurs. It is thus facing a disadvantage in its competition against cbase product for the addition of an EO molecule.
A second problem of the ethoxylation of TMA to produce choline is due to the nature of the ethylene oxide (EO) reactant. EO is highly reactive, extremely flammable and toxic, and it is rather volatile, having an atmospheric boiling point of about 11° C. EO furnishes its own oxygen for a combustion. Autopolymerisation, with high release of energy, may readily be triggered by a wide variety of factors, even in an inert atmosphere. The reaction is rather impossible to control, usually associated with an explosion. Separate vapour phases containing EO as part of the process are therefore preferably avoided. High partial pressures of EO in such vapour phases should definitely be avoided because of the explosion risk.
A third problem is due to the nature of the choline hydroxide product. Because of the strong basic nature of choline hydroxide, the molecule is prone to side product formation via O-ethoxylation and to colour formation and degradation, for example due to Hofmann elimination during the synthesis.
Because choline hydroxide has a base strength similar to NaOH, it is able to activate its own hydroxyl groups, resulting in an important competition between N- and O-ethoxylation during the synthesis reaction. In the case of N-ethoxylation, a TMA molecule reacts with an ethylene oxide molecule, resulting in the desired choline molecule. In the case of O-ethoxylation, the hydroxyl group of a choline molecule reacts again, with one or more other EO molecules, resulting in choline-like molecules having a higher degree of ethoxylation. The O-ethoxylated by-products still behave as a base, but have lower strength and a higher molecular weight. In many applications they represent impurities in the final product. Furthermore, in many applications, such as the production of choline salts, the molarity (usually expressed in mole/liter) of the hydroxide ion is important and therefore each molecule of EO spent on O-ethoxylation represents an economical loss. The degree of formation of O-ethoxylated products which is observed during the choline hydroxide synthesis may be dependent on the base strength of the solution, and hence upon the hydroxide (here primarily the choline hydroxide) concentration. Apart from the concentration, undesired O-ethoxylation may also be increased by higher reaction temperatures.
Furthermore, choline hydroxide is known to be unstable and to develop colour during synthesis and storage, due to decomposition. Decomposition may occur via a so-called Hofmann elimination, resulting in the formation of TMA and acetaldehyde. Liberated TMA leads to odour problems, such as explained above for unreacted TMA left in the choline product. Acetaldehyde ultimately leads to heavily coloured condensation products, causing concentrated choline hydroxide solutions to become brown and black in a matter of a few days at room temperature. Hofmann elimination reactions are favoured by higher temperatures, and the temperature is therefore preferably kept low during the synthesis of choline hydroxide, in order not to obtain a product already heavily coloured immediately after its preparation.
Colour formation is often prevented by the use of very low process temperatures, as low as in the range of 0° C. to 30° C. Although the reaction between TMA and ethylene oxide is strongly exothermic, the liberated reaction heat can at such low temperatures not be recovered efficiently and economically. Furthermore, keeping the reaction temperature of this exothermic reaction below 40° C. poses a challenge in a large scale process, as the temperature of ambient cooling water is usually insufficiently low and the use of powerful and costly cooling equipment would be required. So, the use of lower reaction temperatures requires an additional input of energy instead of a recovery of reaction heat. Moreover, to guarantee acceptable colour over a prolonged period of time during storage, a stabilizer is often added to the choline hydroxide solution after production.
DD 241596 A1 is concerned with avoiding the flash back of the reactor pressure into the EO railcar container. The document discloses how, using pump-around reactors, in a first reaction step a 25-, 40-, or 50% aqueous TMA solution, from one particular vessel selected from a battery of similar vessels, is reacted with gradually added EO in a primary reaction loop at a temperature of 50-60° C., during which the TMA concentration reduces and the cbase concentration increases, until 80-95% of the required EO has been administered. The further conversion of the remaining TMA is performed by circulating the content of that same vessel over a secondary reaction loop, whereby the temperature is kept at 10-15° C., preferably 12° C., under further addition of EO. The excessive EO which may be present in a small amount is subsequently removed by a short application of a vacuum. The reaction of DD 241596 A1 starts with an at least 25% weight TMA solution, which leads after reaction in the first step to a cbase solution of at least 35.7% wt, and after the second step to a cbase solution of at least 40% wt after removal of the excess EO. This two-step batch process leaves something to be desired in terms of by-product and colour formation at the high cbase concentrations practiced in both steps, and in terms of efficient use of reaction volume and energy.
Thus, there remains a need for an effective and efficient process with efficient and low cost heat control and efficient heat recovery for producing choline hydroxide without undesired by-products and colour formation.